Do not return an error if there's no ports to upgrade

e05bba0 added a "Nothing to upgrade"
message when doing a 'port upgrade outdated' and returned an error.
This changes the return value to a non error.

closes https://trac.macports.org/ticket/58309

src/port/port.tcl

@@ -310,7 +310,7 @@ proc require_portlist { nameportlist {is_upgrade "no"} } {
             # "upgrade", let's print a message that's a little easier to
             # understand and less alarming.
             ui_msg "Nothing to upgrade."
-            return 1
+            return 0
         }
         ui_error "No ports matched the given expression"
         return 1